AD7176-2 has unknown noise in sample data.

Hi ADI,

I got EVAL-AD7176-2SDZ (Rev-A), and I use it to test audio signal.

I use AD7176-2 to sample a pure 12.5 kHz 0.1 Vrms sine wave, and sample rate is 125kHz.

But there is a stable low frequency noise as below picture, so it make the wave envelope is unstable.

I

My setting as below:

The attachment is my test data.

Do you have any idea about this problem?

Thanks.

Homeway

12.5kHz.csv.zip
  • 0
    •  Analog Employees 
    on Nov 13, 2017 5:52 PM

    Hi,

    May I ask for your input configurations? How did you connected your AC input voltage? Are you operating at single or dual supply? Can you also try to lower down the number of samples, let say try to make it 20 or just try to zoom in data to take a look at one or two cycles only. As I've seen the voltage values it looks like it swing to +/-140mV which is equivalent to your 0.1Vrms. Can you also checked your inputs at scope and compared it with the ADC results?

    Thanks,

    Jellenie

  • 0
    •  Analog Employees 
    on Nov 13, 2017 8:44 PM

    Hi,

    According to the nyquist theorem, the sampling rate must be at least twice the highest input frequency component. It states the minimum sampling rate to be able to replicate the signal, however, please take note that the higher the value of the sampling rate the closer the output to the expected results as you have more data points therefore the lesser the quantization error. Please take note also that the part requires a settling time at the very first conversion so it has different Tsettle at the first conversion and then the succeeding conversion will occur at the selected ODR.

    Thanks,

    Jellenie

  • 0
    •  Analog Employees 
    on Nov 13, 2017 11:41 PM

    Hi,

    Regarding the number of samples. Same thing can be applied to the number of samples, the minimum number of samples needed to replicate the signal is at least twice the ratio of your ODR/input frequency since the part needs the filter settling time at the very first conversion. Regarding the decrease in voltage, we have a  Virtual Eval on ADI site that may help you evaluate and understand the performance of the AD7176 with any signal frequency or configurations and settings you want. If you take a look at the filter profile of the said tool you will see the attenuation at your operating frequency. At 250ksps ODR, you have -0.15dB, at 125ksp you have -0.26db with12.5KHz input frequency. I think this tool will help you answer to your question.

    Thanks,

    Jellenie

  • Hi Jellenie,

    Thanks for your reply.

    May I ask for your input configurations?

    [Source : Audio precision AP515, single-end 12.5kHz 0.1Vrms sine-wave] 

    How did you connected your AC input voltage?C

    [connector J8 ,  AIN0 as AIN+, AIN1 as AIN- for channel 0]

    Are you operating at single or dual supply?

    [Single power supply for AD7176-2 J4, AVSS = -2.5V]  

    Can you also try to lower down the number of samples, let say try to make it 20 or just try to zoom in data to take a look at one or two cycles only. As I've seen the voltage values it looks like it swing to +/-140mV which is equivalent to your 0.1Vrms. Can you also checked your inputs at scope and compared it with the ADC results?

    [After check with scope, I find that if the sample rate is not enough, then the envelope is not fix.

    For 12.5kHz signal, sample rate should be more than 200 kHz, and capture data will be stable. 

    I am a little confused.

    According to Nyquist theorem, sample rate just need to twice as input signal frequency. 

    Why should I have to use sample rate that more than twice to get stable sample?]

    ==========================================================

    And I have a new question about AD7176-2 sample rate 250 kHz.

    Sample point # of 1 cycle = Sample rate / Signal frequency. =250 kHz / 12.5 kHz = 20

    But you can see below picture, I sample 40 points, but it seems have 3 cycles. 

    The problem only happens at sample rate 250 kHz.

    Could you help to check this issue?

     

    Thanks a lot.

    B.R.

    Homeway

     

     

  • Hi Jellenie,

    Understood.

    Thanks for your reply.

    So higher sample rate will make the result realize.

    But how about the problem about 250 kHs?

    I think the sample data frequency is 1.5 times more than input signal.

    And the voltage is also influenced by sample rate.

    Below is my measurement.

      

    You can see if sample rate decrease, then amplitude decrease, too.

    Thanks.

    B.R.

    Homeway